There is a projected global shortage for benzene which is needed in the manufacture of key petrochemicals such as styrene, phenol, nylon and polyurethanes, among others. Generally, benzene and other aromatic hydrocarbons are obtained by separating a feedstock fraction which is rich in aromatic compounds, such as reformates produced through a catalytic reforming process and pyrolysis gasolines produced through a naphtha cracking process, from non-aromatic hydrocarbons using a solvent extraction process.
In an effort to meet growing world demand for benzene and other aromatics, various industrial and academic researchers have been working for several decades to develop catalysts and processes to make light aromatics (benzene, toluene, xylenes, or BTX) from cost-advantaged, light paraffin (C1-C4) feeds. Prior-art catalysts devised for this application usually contain an acidic zeolite material such as ZSM-5 and one or more metals such as Pt, Ga, Zn, Mo, etc. to provide a dehydrogenation function. For example, U.S. Pat. No. 7,186,871 describes a process for lower alkane aromatization utilizing a Pt/ZSM-5 catalyst. In this patent, data are presented for propane aromatization over various Pt/ZSM-5 catalysts, with the results showing better activity and selectivity to BTX as the Pt level of the catalyst is increased from 0.06 to 0.33% wt. As another example, U.S. Pat. No. 4,350,835 covers a process for producing aromatic compounds from ethane-containing feed using a Ga-containing zeolitic catalyst which may be of the ZSM-5 type. This latter patent teaches that the Ga content of the catalyst may vary between 0.05 and 10% wt, preferably between 0.5 and 5% wt.
Aromatization of ethane and other lower alkanes is thermodynamically favored at high temperature and low pressure without addition of hydrogen to the feed. Unfortunately, these process conditions are also favorable for rapid catalyst deactivation due to formation of undesirable surface coke deposits which block access to the active sites.
For many hydrocarbon processing applications, one approach to reducing catalyst performance decline rates due to coking is to increase the catalyst metals loading in an effort to promote faster hydrogenation/breakup of large coke precursor molecules on the surface. Another approach involves incorporation of additives such as phosphate or rare earths to moderate surface acidity and reduce coking rates under reaction conditions. These approaches are appropriate for processes featuring fixed or slowly-moving catalyst beds wherein the average catalyst particle residence time in the reactor zone between regenerations (coke burnoff steps) is relatively long (at least several days). For example, see U.S. Pat. Nos. 4,855,522 and 5,026,937, which describe ZSM-5-type lower-alkane aromatization catalysts promoted with Ga and additionally containing either a rare earth metal or a phosphorus-containing alumina, respectively.
Yet another approach to circumvent this problem is to devise a lower alkane aromatization process in which the catalyst spends a relatively short time (less than a day) under reaction conditions before being subjected to coke burnoff and/or other treatment(s) aimed at restoring all or some of the original catalytic activity. An example of such a process is one featuring two or more parallel reactors containing fixed or stationary catalyst beds, with at least one reactor offline for catalyst regeneration at any given time, while the other reactor(s) is/are processing the lower alkane feed under aromatization conditions to make aromatics. Another example of such a process features a fluidized catalyst bed, in which catalyst particles cycle rapidly and continuously between a reaction zone where aromatization takes place and a regeneration zone where the accumulated coke is burned off the catalyst to restore activity. For example, U.S. Pat. No. 5,053,570 describes a fluid-bed process for converting lower paraffin mixtures to aromatics.
Requirements for optimal catalyst performance in a process involving a relatively short period of catalyst exposure to reaction conditions between each regeneration treatment, such as a fluidized-bed process, can differ from those of fixed- or moving-bed processes which require longer catalyst exposure time to reaction conditions between regeneration treatments. Specifically, in processes involving short catalyst exposure times, it is important that the catalyst not exhibit excessive initial cracking or hydrogenolysis activity which could convert too much of the feedstock to undesirable, less-valuable byproducts such as methane.
Certain metals such as Pt which are very suitable for catalyzing the dehydrogenation reactions needed for aromatization can also, under certain circumstances, display undesirable hydrogenolysis activity that leads to excessive production of methane from higher hydrocarbons. The inclusion of a second, inert or less-active metal in a catalyst composition to help suppress the hydrogenolysis activity of the first, more-active metal is used in commercial scale catalytic naphtha reforming in which C5-C12 paraffins and naphthenes are converted to aromatic compounds with catalysts which are predominantly bimetallic and are supported on chloride-promoted alumina. As indicated in a catalytic naphtha reforming review article by C. A. Querini in volume 6, pages 1-56 of the Encyclopedia of Catalysis (I. T. Horvath, ed.; published by John Wiley & Sons, Inc., Hoboken, N.J., USA, 2003), these catalysts typically contain Pt plus another metal such as Re (in sulfided form) or Sn. Among other effects, these second metals can interact with the Pt to reduce hydrogenolysis activity, thereby decreasing the rate of unwanted methane formation.
These Pt/Re and Pt/Sn catalysts, supported on chloride-promoted alumina, are widely employed in fixed-bed (semi-regenerative) and moving-bed (continuous) naphtha reformers, respectively, and their compositions are optimized for relatively long catalyst exposure times to reaction conditions between regeneration treatments. The average catalyst particle residence time in the reaction zone between regeneration treatments ranges from a few days in moving bed reactors and up to 1 or 2 years in fixed bed reactors. According to the article by Querini mentioned above, typical Pt and Sn levels in Pt/Sn naphtha reforming catalysts are about 0.3% wt each. Such catalysts, which usually lack a strongly acidic zeolite component, do not work well for lower alkane aromatization.
It would be advantageous to provide a light hydrocarbon dehydroaromatization process which can be performed under conditions thermodynamically favorable for light alkane aromatization as described above, which provides for relatively short catalyst exposure time to reaction conditions, wherein the average catalyst particle residence time in the reaction zone between regeneration treatments may be from about 0.1 second to about 30 minutes in a fluidized bed reactor and from a few hours up to a week in moving bed and fixed bed reactors, and in which the catalyst composition is optimized to reduce excessive initial production of less-desirable byproducts such as methane.